[ELF] Refine warning condition for memory region assignment for non-allocatable section

The warning "ignoring memory region assignment for non-allocatable section"  should be generated under the following conditions:

* sections without SHF_ALLOC attribute and,
* presence of input sections or data commands (ByteCommand)

The goal of the change is to reduce spurious warnings that are generated for some output sections that have no input section.

Reviewed By: MaskRay, peter.smith

Differential Revision: https://reviews.llvm.org/D151802